What should the following do:
(defun foo (a s v)
(progv s v (print a)))
with:
(foo 3 '(a) '(4))
The actual result is 3 but from the book description of PROGV
on p112 it appears that it should be 4. I have tried this on
3 different common lisps with the same result.
The `a' in (print a) refers to the lexical variable `a' named in
foo's parameter list. The progv binds the dynamic variable `a'.
You can get the answer to be 4 a number of ways:
1.
(defvar a)
(defun foo (a s v)
(progv s v (print a)))
This makes all references to `a' pervasively dynamic so even
the call to foo binds the dynamic `a'.
2.
(defun foo (a s v)
(declare (special a))
(progv s v (print a)))
Has the same effect on foo, but other functions can use a lexically
scoped `a'.
3.
(defun foo (a s v)
(progv s v
(declare (special a))
(print a)))
The binding of `a' during the call to foo is lexical, but the
reference in (print a) is dynamic.